Antenna steering for an access point based upon probe signals

ABSTRACT

A method for operating an access point in a wireless local area network (WLAN) is provided. The access point includes a directional antenna for communicating with a plurality of remote stations, and the directional antenna includes an omni angle and a plurality of directional angles. The method determines a preferred direction to the point the directional antenna from the access point to a desired remote station. This determination is based on a sequence of probe signals exchanged between the access point and the remote station. The probe response signals are measured by the access point, and are stored in an antenna database.

RELATED APPLICATION

This application claims the benefit of U.S. provisional application Ser.No. 60/479,701, filed Jun. 19, 2003, the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of wireless local areanetworks, and in particular, to an antenna steering algorithm for anaccess point operating within a wireless local area network.

BACKGROUND OF THE INVENTION

Various standards allow remote stations, such as portable computers, tobe moved within a wireless local area network (WLAN) and connect viaradio frequency (RF) transmissions to an access point (AP) that isconnected to a wired network. The wired network is often referred to asa distribution system. The various standards include the IEEE 802.11standard and its corresponding letter revisions thereof, such as 802.11band 802.11g, for example.

A physical layer in the remote stations and in the access point provideslow level transmissions by which the stations and the access pointcommunicate. Above the physical layer is a media access control (MAC)layer that provides services, such as authentication, deauthentication,privacy, association and disassociation, for example.

In operation, when a remote station comes on-line, a connection is firstestablished between the physical layers in the station and the accesspoint. The MAC layers can then connect. Typically, for the remotestations and the access point, the physical layer RF signals aretransmitted and received using monopole antennas.

A monopole antenna radiates in all directions, generally in a horizontalplane for a vertically oriented element. Monopole antennas aresusceptible to effects that degrade the quality of communication betweenthe remote station and the access point, such as reflection ordiffraction of radio wave signals caused by intervening objects.Intervening objects include walls, desks and people, for example. Theseobjects create multi-path, normal statistical fading, Rayleigh fading,and so forth. As a result, efforts have been made to mitigate signaldegradation caused by these effects.

One technique for counteracting the degradation of RF signals is to usetwo antennas to provide diversity. The two antennas are coupled to anantenna diversity switch in one or both of the remote stations and theaccess point. The theory behind using two antennas for antenna diversityis that, at any given time, at least one of the antennas is likelyreceiving a signal that is not suffering from the effects of multi-path.Consequently, this antenna is the antenna that the remote station oraccess point selects via the antenna diversity switch fortransmitting/receiving signals. Nonetheless, there is still a need toaddress the degradation of RF signals between the remote stations and anaccess point in a wireless local area network.

SUMMARY OF THE INVENTION

In view of the foregoing background, an object of the present inventionis to improve communications between an access point and remote stationswithin a wireless local area network.

An improvement over simple diversity is provided through an antennasteering process for access points (i.e., wireless gateways) used inwireless local area networks. Directional antennas improve thethroughput of the network, and increase the range between the accesspoint and the remote stations (i.e., wireless user devices). Adirectional antenna provides a higher signal-to-noise ratio than anomni-directional antenna in most cases, thus allowing the link tooperate at higher data rates.

The antenna steering process may be resident in the media access control(MAC) layer of the access point, and selects a best or preferreddirectional antenna pattern based on signal quality metrics availablefrom the physical layer upon receiving signals from the remote stations.

According to the principles of the present invention, during processessuch as registration, authentication or subsequent data exchangesbetween the access point and a selected remote station, a preferreddirection for the steered access point antenna is determined. In oneembodiment, software or firmware operating at the access point makesthis determination. The access point antenna control software/firmwaremay build a database that includes the identity of the remote stationand the antenna direction associated with that station for achievingoptimum communications performance.

Hardware may be employed to operate with inherent diversity selectioncircuitry in typical 802.11 equipment for selecting the preferreddirectional antenna angle. The access point may use signaling to causethe remote stations to transmit a probe response signal, wherein theaccess point measures the signal quality of the probe response signal.The access point may compare metrics corresponding to signals receivedfrom the remote stations in a directional antenna mode against metricscorresponding to signals received from the remote stations in anomni-directional mode to determine if a new antenna scan should beperformed. If the access point determines that hidden nodes are present,it may invoke a protection mechanism using request-to-send/clear-to-send(RTS/CTS) messaging as defined in the 802.11 standard, for example.

The benefits of augmenting the access point with a directional antennaare two-fold: improved throughput to individual remote stations and anability to support more users in the network. In most RF environments,the signal level received at the remote station can be improved byhaving the access point transmit using a shaped antenna beam pointed inthe direction of the station. The shaped antenna beam may provide a 3–5dB gain advantage, for example, over the omni-directional antennatypically deployed with an access point. The increased signal levelallows the link between the access point and the remote station tooperate at higher data rates, especially at the outer band of thecoverage area. The directional antenna steering process is resident inthe access point to support operation with the remote stations.

More particularly, the present invention is directed to a method foroperating an access point in a WLAN, with the access point comprising adirectional antenna for communicating with a plurality of remotestations. The directional antenna comprises an omni angle and aplurality of directional angles. The method comprises selecting one ofthe remote stations from the plurality of remote stations, transmittinga first probe signal via the omni angle of the directional antenna tothe selected remote station, and measuring a first probe response signalreceived via the omni angle from the selected remote station respondingto the first probe signal.

The method further comprises transmitting a respective second probesignal via each one of the plurality of directional angles of thedirectional antenna to the selected remote station, and measuring asecond probe response signal received via each directional angle fromthe selected remote station responding to the respective second probesignal. The measured first probe response signal and the respectivemeasured second probe response signals from the selected remote stationare stored in an antenna database.

The method may further comprise selecting a preferred directional anglefor the selected remote station based upon the measured second proberesponse signals, and comparing the measured first probe response signalfrom the omni angle with the measured second probe response signal fromthe preferred directional angle. The omni angle or the preferreddirectional angle may be selected based upon the comparing forcontinuing communications with the selected remote station. Thepreferred directional angle may be selected if the measured signalassociated therewith exceeds the measured signal associated with theomni angle by a predetermined threshold.

The method may further comprise selecting a next remote station from theplurality of remote stations, repeating the transmitting of the firstand second probe signals to the next selected remote station, and themeasuring of the first and second probe response signals received fromthe next selected remote station. The measured first probe responsesignal and the respective measured second probe response signals fromthe next selected remote station are stored in the antenna database. Theselecting, the transmitting and the storing are repeated for each of theremaining remote stations.

The first probe signal may comprise a request-to-send (RTS) message andthe first probe response signal may comprise a clear-to-send (CTS)message. Similarly, the second probe signal may comprise an RTS messageand the second probe response signal may comprise a CTS message.

The measuring may comprise determining at least one of a received signalstrength indication, a carrier-to-interference ratio, an energy-per-bitratio and a signal-to-noise ratio. Selection of the omni angle andscanning through the plurality of directional angles may be performed atthe media access control (MAC) layer of the access point.

The method may further comprise updating the antenna database for theselected remote station if there is no communications between the accesspoint and the selected remote station for a certain period of time. Theupdating may comprise repeating the transmitting of the first and secondprobe signals to the selected remote station, and the measuring of thefirst and second probe response signals received from the selectedremote station.

The access point may be operating based upon the IEEE 802.11 standard orthe IEEE 802.16 standard. The directional antenna may comprise at leastone active element and a plurality of passive elements. Another aspectof the present invention is directed to an access point comprising adirectional antenna comprising an omni angle and a plurality ofdirectional angles, and a controller connected to the directionalantenna for control thereof.

The controller selects one of the remote stations from the plurality ofremote stations, transmits a first probe signal via the omni angle ofthe directional antenna to the selected remote station, and measures afirst probe response signal received via the omni angle from theselected remote station responding to the first probe signal. Thecontroller further transmits a respective second probe signal via eachone of the plurality of directional angles of the directional antenna tothe selected remote station, measures a second probe response signalreceived via each directional angle from the selected remote stationresponding to the respective second probe signal, and storing in anantenna database the measured first probe response signal and therespective measured second probe response signals from the selectedremote station.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings. The drawings are not necessarily to scale,with emphasis instead being placed on illustrating the principles of theinvention.

FIG. 1A is a schematic diagram of a wireless local area network (WLAN)employing the principles of the present invention;

FIG. 1B is a schematic diagram of an access point in the WLAN of FIG. 1Aperforming an antenna scan;

FIG. 2A is a view of an access point of FIG. 1A having an externaldirective antenna array;

FIG. 2B is a view of the access point of FIG. 2A having the directiveantenna array incorporated in an internal PCMCIA card;

FIG. 3A is a view of the directive antenna array of FIG. 2A;

FIG. 3B is a schematic diagram of a switch used to select a state of anantenna element of the directive antenna of FIG. 3A;

FIG. 4 is a block diagram of an access point of FIG. 1A employingsubsystems, layers and an antenna steering process according to theprinciples of the present invention;

FIG. 5A is a signal diagram optionally used by the antenna steeringprocess of FIG. 4;

FIG. 5B is an alternative signal diagram optionally used by the antennasteering process of FIG. 4;

FIG. 6 is an alternative block diagram of FIG. 4 in which antennadiversity circuits are employed;

FIG. 7 is a signal diagram using a hidden node technique optionally usedby the antenna steering process of FIG. 4;

FIG. 8 is a top view of the network of FIG. 1 with bi-directionalsignaling;

FIG. 9 is a top view of the network of FIG. 1 with indications of theantenna beams;

FIG. 10 is a flowchart of a method for operating an access point in aWLAN based upon spatial diversity in accordance with the presentinvention;

FIG. 11 is a flowchart of a method for operating an access point in aWLAN based upon probe signals in accordance with the present invention;

FIGS. 12 and 13 are respective flowcharts of a method for operating anaccess point in a WLAN based upon control frames in forward and reverselinks in accordance with the present invention; and

FIG. 14 is a flowchart of a method for operating an access point in aWLAN based upon hidden node recognition in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime notation is used toindicate similar elements in alternate embodiments.

Referring initially to FIG. 1A, a wireless local area network (WLAN) 100having a distribution system 105 will initially be discussed. Accesspoints 110 a, 10 b and 110 c are connected to the distribution system105 via wired connections, such as wired data network connections. Eachof the access points 110 a, 110 b and 110 c has a respective zone 115 a,115 b, 115 c in which it is capable of communicating via radio frequency(RF) signals with the remote stations 120 a, 120 b, 120 c. The remotestations 120 a, 120 b, 120 c are supported with wireless local areanetwork hardware and software to access the distribution system 105. Inthe following description, when a general reference is made to theaccess points, the remote stations and the zones, the respectivereference numerals 110, 120 and 115 may be used.

Present technology provides the access points 110 and the remotestations 120 with antenna diversity. Antenna diversity allows the accesspoints 110 and the remote stations 120 to select one of two antennas toprovide transmit and receive duties based on the quality of signalsbeing received. One reason for selecting one antenna over the otheroccurs in the event of multi-path fading, in which a signal taking twodifferent paths causes signal cancellation to occur at one antenna butnot the other. Another example is when interference is caused by twodifferent signals received at the same antenna. Yet another reason forselecting one of the two antennas is due to a changing environment, suchas when a remote station 120 c is carried from the third zone 115 c tothe first or second zones 115 a, 115 b as indicated by arrow 125.

FIG. 1B is a block diagram of a subset of the network 100 illustrated inFIG. 1A in which an access point 110 b, employing the principles of thepresent invention, is shown in greater detail with respect to thedirective antenna lobes 130 a–130 i. The directive antenna lobes 130a–130 i will also be generally indicated by reference numeral 130. Theaccess point 110 b sequences through the antenna lobes 130 during a scanof its environment to determine a preferred antenna direction.

During a scan, the access point 110 b uses a directive antenna, as shownin greater detail in FIGS. 2A and 2B, to scan in search of RF signalstransmitted by the remote station 120 b. At each scan direction (i.e.,angle or antenna pattern), the access point 110 b measures a signal orprobe response and calculates a respective metric for that scan angle.Examples of the metrics include a received signal strength indication(RSSI), a carrier-to-interference ratio (C/I), an energy-per-bit ratio(Eb/No), or other suitable measures, such as a signal-to-noise ratio(SNR), of the quality of the received signal or signal environment. Acombination of these measurements may also be made to determine the bestor preferred antenna pattern, as readily appreciated by those skilled inthe art. Based on the measured signal quality metrics, the access point10 b determines the preferred antenna angle or direction forcommunicating with the remote station 120 b.

The scans may occur before or after the remote station 110 b has beenauthenticated and has associated with the distribution system 105. Thus,the initial antenna scan may be accomplished within the MAC layer.Alternatively, the initial scan may be accomplished external from theMAC layer. Similarly, scans occurring after the remote station 110 b hasauthenticated and has associated with the distribution system 105 may beaccomplished within the MAC layer or by processes occurring external theMAC layer.

FIG. 2A is a diagram of an access point 110 using an external directiveantenna array 200 a. The directive antenna array 200 a includes fivemonopole passive antenna elements 205 a, 205 b, 205 c, 205 d and 205 eand one monopole, active antenna element 206. The passive antennaelements 205 a, 205 b, 205 c, 205 d and 205 e are generally referred tobelow by reference numeral 205. The directive antenna element 200 a isconnected to the access point 110 via a universal serial bus (USB) port215. Other types of connections between the directive antenna array 200a and the access point 110 are readily acceptable.

The passive antenna elements 205 in the directive antenna array 200 aare parasitically coupled to the active antenna element 206 to permitscanning. By scanning, it is meant that at least one antenna beam of thedirective antenna array 200 a can be rotated, optionally 360 degrees, inincrements associated with the number of passive antenna elements 205.

A detailed discussion of the directive antenna array 200 a is providedin U.S. Patent Publication No. 2002/0008672, published Jan. 24, 2002,entitled “Adaptive Antenna For Use In Wireless Communications System”,the entire disclosure of which is incorporated herein by reference andwhich is assigned to the current assignee of the present invention.Example methods for optimizing antenna direction based on received ortransmitted signals by the directive antenna array 200 a are alsodiscussed therein.

The directive antenna array 200 a may also be used in anomni-directional mode to provide an omni-directional antenna pattern.The access points 110 may use an omni-directional pattern fortransmission or reception. The access points 110 may also use theselected directional antenna when transmitting to and receiving from theremote stations 120.

FIG. 2B is an isometric view of an access point 110 with an internaldirective antenna 220 b. In this embodiment, the directive antenna array200 b is on a PCMCIA card 220. The PCMCIA card 220 is carried by theaccess point 110 and is connected to a processor (not shown). Thedirective antenna array 200 b provides the same functionality as thedirective antenna array 200 a illustrated in FIG. 2A.

It should be understood that various other forms of directive antennaarrays can be used. Examples include the arrays described in U.S. Pat.No. 6,515,635 issued Feb. 4, 2003, entitled “Adaptive Antenna For Use InWireless Communication Systems” and U.S. Patent Publication No.2002/0036586, published Mar. 28, 2002, entitled “Adaptive Antenna ForUse In Wireless Communication System,” the entire teachings of which areincorporated herein by reference and which are assigned to the currentassignee of the present invention.

FIG. 3A is a detailed view of the directive antenna array 200 a thatincludes the passive antenna elements 205 and the active antenna element206 as discussed above. The directive antenna array 200 a also includesa ground plane 330 to which the passive antenna elements areelectrically coupled, as discussed below in reference to FIG. 3B.

Still referring to FIG. 3A, the directive antenna array 200 a provides adirective antenna lobe 300 angled away from antenna elements 205 a and205 e. This is an indication that the antenna elements 205 a and 205 eare in a reflective mode, and the antenna elements 205 b, 205 c and 205d are in a transmission mode. In other words, the mutual couplingbetween the active antenna element 206 and the passive antenna elements205 allows the directive antenna array 200 a to scan the directiveantenna lobe 300, which, in this case, is directed as shown as a resultof the modes in which the passive elements 205 are set. Different modecombinations of passive antenna element 205 result in different antennalobe 300 patterns and angles, as readily understood by those skilled inthe art.

FIG. 3B is a schematic diagram of an example circuit that can be used toset the passive antenna elements 205 in the reflective or transmissionmodes. The reflective mode is indicated by a representative elongateddashed line 305, and the transmission mode is indicated by a shorteneddashed line 310. The representative modes 305 and 310 are respectivelycaused by coupling to a ground plane 330 via an inductive element 320 ora capacitive element 325. The coupling of the passive antenna element205 a through the inductive element 320 or capacitive element 325 isperformed via a switch 315. The switch 315 may be a mechanical orelectrical switch capable of coupling the passive antenna element 205 ato the ground plane 330. The switch 315 is set via a control signal 335.

Coupled to the ground plane 330 via the inductor 320 is the passiveantenna element 205 a, which is effectively elongated as shown by thelonger representative dashed line 305. This can be viewed as providing a“backboard” for an RF signal coupled to the passive antenna element 205a via mutual coupling with the active antenna element 206. In the caseof FIG. 3A, both passive antenna elements 205 a and 205 e are connectedto the ground plane 330 via respective inductive elements 320. At thesame time, in the example of FIG. 3A, the other passive antenna elements205 b, 205 c and 205 d are electrically connected to the ground plane330 via respective capacitive elements 325.

The capacitive coupling effectively shortens the passive antennaelements as represented by the shorter representative dashed line 310.Capacitively coupling all of the passive elements 325 effectively makesthe directive antenna array 200 a an omni-directional antenna. It shouldbe understood that alternative coupling techniques may also be usedbetween the passive antenna elements 205 and the ground plane 330, suchas delay lines and lumped impedances, for example.

Jumping to FIG. 9, an overhead view of the access point 110 b generatingan omni-directional antenna pattern 905 and a directional antennapattern 910 through use of the directive antenna array 200 a or 200 b isprovided. The access point 110 b communicates with multiple stations 120a–120 d. Since access points 110 are usually remotely installed withoutnearby obstructions or moving reflectors (e.g., high on a wall orceiling), the selection of the preferred antenna pattern direction islikely not going to change throughout the connection with a given remotestation 120.

The illustrated access point 110 b may make use of a directional antenna200 a for downlink data frames transmitted to a selected remote station120 c. For most broadcast and control frames, the access point may usethe omni-directional antenna pattern 905 and the lowest available datarate to ensure that all remote stations 120 receive them. Thedirectional antenna 200 a may not increase the coverage area of thenetwork 100, but may increase the data rate for data frames sent to theremote stations 120. The increased downlink rate is useful because themajority of the data transferred over the network 100 appears on thedownlink (e.g., web page access, file transfers). One option is to useswitched spatial diversity when the access point 110 b is required toreceive in the omni mode. The potential added link margin of 5 dBaccommodates a throughput increase of 300%, for example.

Uplink data frames sent from the selected remote station 120 c to theaccess point 110 b during contention periods (CP) are received using theomni-directional antenna pattern since any remote station may havetransmitted the frame. For large frames, the network configuration mayrequire the remote station to use the request-to-send/clear-to-send(RTS/CTS) mechanism to reserve the wireless medium. In this case, theaccess point 110 b could receive in a directional mode to increase thedata rate on the uplink. This is somewhat dependent on the data rateselection algorithm implemented at the remote station 120 c.

In downlink transmissions, the access point 110 b may decide to transmitsmall packets during contention periods using the omni-directionalpattern and a lower data rate. The reason for this is that a remotestation on the “other” side of the coverage area (such as remote station120 e) may not hear the access point transmission from the directionalantenna pattern 910 pointed away from it. This is the familiar “hiddennode” problem where two remote stations 120 do not hear each other andend up transmitting at the same time. In this case the two remotestations are 120 c and 120 e. A method to avoid this problem, especiallyfor large data frames, is described below in reference to FIG. 7.

The directional antenna patterns at the access point 110 can thusprovide higher data rates for downlink and uplink data frame exchangeswith the remote stations 120, which is the bulk of the network traffic.Network connectivity is maintained with the nominal gain of theomni-directional antenna of the access point 110. That is, the remotestations 120 can associate with the access point 110 and maintain theconnection without the use of the directional antenna 200 a.

A set of rules as provided in TABLE 1 can be defined to take advantageof the omni-directional and directional characteristics of thedirectional antenna 200 a. TABLE 1 includes addresses of the remotestations 120 currently associated with the access point 110 and theircurrent antenna direction selection. TABLE 1 may delineate exampleantenna direction selections based on frame sequences from the 802.11standard (TABLES 21 and 22 therein). In TABLE 1, “Dir” indicatesdirection, “UL” indicates uplink, and “DL” indicates downlink.

TABLE 1 Example Antenna Selection Rules Antenna Sequence Dir SelectionBeacon DL Omni Data DL Dir See FIG. 5A RTS-CTS-Data UL Omni/Dir See FIG.5B

A process can be described in a set of rules that determine when toselect the omni-directional pattern and when to select a directionalpattern. For example, the access point 110 may select a directionalpattern during time intervals when transmitting or receiving to/from asingle remote station 120.

A block diagram showing the interfaces of the access point 110 is shownin FIG. 4. The illustrated access point 110 includes various subsystemsand layers. An antenna subsystem 405 may include the directional antenna200 b and supporting circuitry, buses and software to operate thedirectional antenna. The antenna subsystem 405 interfaces to thephysical layer 410 and provides RF signals 412 thereto.

The physical layer 410 processes the RF signals 412 and determinessignal quality measurements 417 to an antenna steering process 420. Thephysical layer 410 sends processed signals based upon the RF signals 412to the MAC layer 415. The MAC layer 415 generates timing controlmessages 422, which are also sent to the antenna steering process 420 inorder to switch the antenna to the omni mode or directional mode whenrequired.

The MAC layer 415 also sends data frames 429 to other processes (notshown). The illustrated physical layer 410, MAC layer 415 and antennasteering process 420 may reside within a controller 400. The antennasteering process 420 may be stored within a memory, for example, whichmay be a stand-alone memory or an embedded memory within a processor,for example.

The antenna steering process 420 maintains an “antenna table ordatabase” or a “direction table or database” 425 as a function of thereceived signal quality measurements 417 made during antenna scans ofeach remote station 120. For example, the direction table 425 may storea station ID and a corresponding antenna direction (A, B, C) fordirectional communications with the remote stations 120. Once theantenna directions in the direction table 425 have been determined, theantenna steering process 420 is used to provide directional antennacontrol 427 to the antenna subsystem 405. If the signal qualitymeasurements 417 are above a predetermined threshold indicating that thehighest data rate can be supported in the omni-directional mode, theantenna direction may be held at the omni-directional (O) mode.

The following paragraphs describe various techniques in accordance withthe present invention for determining the preferred direction to point adirectional antenna 220 b from an access point 110 to a remote station120. The first technique employs a spatial diversity selectionmechanism. The second technique uses a sequence of probe signalsexchanged between the access point 110 and the remote stations 120. Thethird technique uses control messages (e.g., ACK or CTS) to make signalquality measurements of the received antenna directions at the accesspoint 110. The third technique is applicable in both forward and reversedirection links.

The first technique assumes that current 802.11 devices incorporateantenna switched diversity scan/control and that future 802.11 devices,such as 802.11a/802.11g/802.11n will also support switched diversity.The first technique is applicable after a remote station 120 hasauthenticated and associated itself with a network. It is assumed thatthe initial antenna scan is accomplished within the MAC/network layerprotocol. With a directional or multi-element antenna 220 a, the firsttechnique can make use of the diversity protocol to keep the antennaposition/selection updated.

Referring now to FIG. 6, the first technique functions as follows. Theillustrated access point 110′ includes a controller 600′ connected tothe antenna subsystem 405′. The controller 600′ comprises a physicallayer 410′, which is given access to the antenna control signals, and aMAC layer (FIG. 4). The MAC layer writes antenna selections intoregister A 605 a′ and register B 605 b′. Register A 605 a′ contains theselected antenna position, and register B 605 b′ contains a candidateantenna position. The physical layer 410′ is also in communications witha multiplexer 610′. The physical layer 410′ sends a diversity selectionswitch control signal 607′ to the multiplexer 610′ in a typicaldiversity selection control manner, but in this case, the diversityselection switch control signal controls whether the contents ofregister A 605 a′ or register B 605 b′ are used.

The selected antenna position is initially chosen during the networkauthentication/association protocol. The candidate antenna position isany other antenna position (including an omni-directional mode). Thecandidate antenna position is changed, in a predetermined sequence,after a valid packet has been received or after not receiving anypackets for a predetermined time period.

After successfully receiving a packet, the physical layer 410′ sendsreceived signal quality metrics (signal strength, signal-to-noise ratio,multi-path/equalizer metrics, etc.) for both antenna positions to theMAC layer. During the packet reception, the physical layer 410′functions as it does now for 802.11; that is, to switch between the twoantenna positions and to use the best antenna position for packetreception. After valid packet reception by the physical layer 410′, thesignal quality metrics for the two antenna positions are sent to the MAClayer. The MAC layer updates both the selected antenna position and thecandidate antenna position. The selected antenna position is replacedwith the best position based on the data received from the physicallayer 410′. Filtering/hysteresis may be used to keep from “ping-ponging”between two antenna positions.

As stated previously, this technique takes advantage of the current802.11 antenna switched diversity methods. It should be understood thatthis first technique may include hardware, software/firmware orcombinations thereof.

Referring now to FIG. 10, a flowchart of the above described method foroperating an access point 110 in a WLAN 100 based upon spatial diversitywill be discussed. From the start (Block 1000), the method comprisescommunicating with the remote station 120 using a current angle of thedirectional antenna 220 b at Block 1010. Scanning through a plurality ofalternate angles of the directional antenna 220 b for communicating withthe remote station 120 during the preamble is performed at Block 1020.Respective signals received via the current angle and the plurality ofalternate angles from the remote station 120 are measured at Block 1030.During the preamble, the current angle or one of the plurality ofalternate angles is selected at Block 1040 as a preferred angle basedupon the measured signals for continuing communications with the remotestation 120. The method ends at Block 105.

The second technique is based upon the transmission by the access point110 of RTS messages to the remote stations 120, and the reception of CTSmessages transmitted in response by the remote stations to the accesspoint. The 802.11 standard also defines a probe request/probe responseexchange, which is typically used by remote stations 120 to determinethe quality of the link to other stations 120.

When used by the access point 110 to determine the preferred pointingdirection to a selected remote station 120, as illustrated in FIG. 8,the access point 110 transmits a probe request signal 805 in the omnipattern and each of the potential directional patterns 130, and measuresthe signal quality of the probe response signal 810 sent back from theremote station 110 while operating in the respective patterns.

Measurements of these response frames 810 make this a more reliabletechnique than the diversity selection technique described above. Thissecond technique is preferably employed at least once immediately aftera remote station 120 has associated with the access point 110. However,there is an impact to network efficiency using additional proberequest/probe response signals, but these exchanges may be infrequent.

Referring now to FIG. 11, a flowchart of the above described method foroperating an access point 110 in a WLAN 100 based upon probe signalswill be discussed. From the start (Block 1100), the method comprisesselecting a remote station 120 at Block 1110, transmitting a first probesignal via the omni angle of the directional antenna 220 b to theselected remote station at Block 1120, and measuring a first proberesponse signal received via the omni angle from the selected remotestation responding to the first probe signal at Block 1130.

A respective second probe signal is transmitted at Block 1140 via eachone of the plurality of directional angles of the directional antenna220 b to the selected remote station 120, and a second probe responsesignal received via each directional angle from the selected remotestation responding to the respective second probe signal is measured atBlock 1150. The measured first probe response signal and the respectivemeasured second probe response signals from the selected remote station120 are stored in an antenna database at Block 1160.

A preferred directional angle for the selected remote station 120 isselected at Block 1170 based upon the measured second probe responsesignals. The measured first probe response signal from the omni angle iscompared at Block 1180 with the measured second probe response signalfrom the preferred directional angle. The first probe signal comprises arequest-to-send (RTS) message and the first probe response signalcomprises a clear-to-send (CTS) message. Similarly, the second probesignal comprises an RTS message and the second probe response signalcomprises a CTS message. The omni angle or the preferred directionalangle is selected at Block 1190 based upon the comparing for continuingcommunications with the selected remote station 120. The method ends atBlock 1195.

The third technique exploits the control frames used in normal dataexchanges between the access point 110 and the remote stations 120. Thistechnique may be used in both forward link communications and reverselink communications. Since the clear-to-send (CTS) and acknowledge (ACK)messages are sent at the lower data rates, the access point 110 can usethese messages to compare the omni pattern 905 to the currently selecteddirectional pattern 130. This is illustrated in FIG. 5A with the dashedlines on the antenna selection timing. This can serve as a method todetermine whether the currently selected direction 130 has maintainedits advantage over the omni-directional pattern 905. This advantage istypically based upon a predetermined threshold to prevent frequentswitching between two antenna patterns having similar signal qualitymetrics.

For example, during the CTS messages, the omni-directional mode may beused to receive this message to calculate a first signal qualitymeasurement. During the ACK message, a test antenna direction may beused to receive this message to calculate a second signal qualitymeasurement. Comparison of the first and second signal qualitymeasurements is performed and a determination is made as to whether thetest antenna direction should be stored. That is, whether thedirectional mode provides a higher gain than omni-directional mode.Comparisons may also be performed between two different directionalantenna directions.

The same types of measurements and comparisons may be conducted during areverse link data transmission, as shown in FIG. 5B. During the ACKmessage, the access point 110 may calculate a signal quality measurementand compare it to an omni-directional mode measurement or otherdirectional mode measurement. Comparisons may be conducted over severalcommunications with the selected remote station 110 before scanning adifferent antenna direction.

The direction table 425 in FIG. 4 may be augmented with signal qualitymeasurements from the process or processes described above for the omniand selected directional antenna pattern. If the advantage drops below apredetermined threshold, the access point 110 reverts back to the omniselection and performs an antenna search using one of the first twotechniques described above.

In cases where the remote station 120 goes into a power-save mode or haslong idle periods with no data transfers, the access point 110 revertsback to the omni pattern selection. When the remote station 120 becomesactive again, the access point 110 may perform another antenna search.

Referring now to FIGS. 12 and 13, respective flowcharts of a method foroperating an access point 120 in a WLAN 100 based upon control frames inforward and reverse links will be discussed. From the start (Block1200), the method comprises receiving in the forward link a firstcontrol frame via a first antenna pattern of the directional antenna 220b from the remote station 120 at Block 1210, and transmitting a firstdata frame to the remote station at Block 1220, and receiving a secondcontrol frame via a second antenna pattern of the directional antennafrom the remote station at Block 1230. A signal quality of the firstcontrol frame received via the first antenna pattern and a signalquality of the second control frame received via the second antennapattern are measured at Block 1240. The respective measured signalqualities associated with the first and second antenna patterns arecompared at Block 1250. The second antenna pattern for transmitting asecond data frame to the remote station 120 is selected at Block 1260 ifthe measured signal quality associated with the second antenna patternexceeds the measured signal quality associated with the first antennapattern by a predetermined threshold. The first control frame receivedcomprises a clear-to-send message, and the second control frame receivedcomprises an acknowledgement message. The method ends at Block 1270.

The method for operating an access point 120 in a WLAN 100 based uponcontrol frames in the reverse link comprises from the start (Block1300), receiving a first control frame via a first antenna pattern ofthe directional antenna 220 b from the remote station at Block 1310,transmitting a second control frame to the remote station at Block 1320,and receiving a first data frame via a second antenna pattern of thedirectional antenna from the remote station at Block 1330. A signalquality of the first control frame received via the first antennapattern and a signal quality of the first data frame received via thesecond antenna pattern are measured at Block 1340. The respectivemeasured signal qualities associated with the first and second antennapatterns are compared at Block 1350. The second antenna pattern fortransmitting a second data frame by the access point 110 to the remotestation 120 is selected at Block 1360 if the measured signal qualityassociated with the second antenna pattern exceeds the measured signalquality, associated with the first antenna pattern by a predeterminedthreshold. The first control frame received comprises a request-to-sendmessage, and the second control frame transmitted comprises aclear-to-send message. The method ends at Block 1370.

The fourth techniques is a hidden node protection technique thatprovides a protection mechanism when employing a directional antenna 220b at the access point 110 to reduce or eliminate the occurrence ofhidden nodes. Hidden nodes occur when not all of the remote stations 120in the network 100 can hear communications between the access point 110and a selected remote station 120, and therefore, those that cannot hearcan transmit when the medium is in use. This causes collisions,particularly at the access point 110.

When the access point 110 has data for transmission to a remote station120, the control process sets the selected antenna direction by scanningthe direction table 425 in FIG. 4 to determine if there are potentialhidden nodes. For example, the access point 110 may look for remotestations 120 in the opposite direction from the selected antennadirection.

Referring to the timing diagram of FIG. 7, if the control softwaredetermines that a potential for hidden nodes exists, the access point110 first transmits a CTS message to a known unused MAC address usingthe omni-directional mode of the antenna 220 a. This process serves totell all of the remote stations 120 in the network that an exchange isto occur and not to transmit until the exchange is finished. The accesspoint 110 then switches to the selected antenna direction for theintended remote station 120 and communications proceed. Another approachto preventing the hidden node problem is to perform a four-way frameexchange protocol (RTS, CTS, data and ACK) with a desired remote station120.

If the control software determines that there is no potential for ahidden node, the access point 110 may not send the CTS message andcommunications may start immediately with the access point 110 antennaset to the proper direction. If required by the network protocol, theRTS message can be addressed to the intended receiver, resulting in aCTS message back to the access point 110 as an acknowledgement, as shownin FIG. 5A.

Note that in the process described in reference to FIG. 7, efficiency isimproved since the RTS message is not transmitted by the access point110 since the CTS message is all that is necessary to cause the remotestations 120 to halt transmissions. The remote station 120 indicated inthe ID section of the standard 802.11 protocol header ensures thespecified remote station receives the data frame.

Referring now to FIG. 14, a flowchart for operating an access point 120in a WLAN 100 based upon hidden node recognition will be discussed. Fromthe start (Block 1400), the method comprises creating an antennadatabase by associating between the access point 110 and each remotestation 120 a respective measured signal quality corresponding to theplurality of antenna patterns at Block 1410. The respective measuredsignal qualities are determined by the access point 110 based uponcommunications with each remote station 120. For each remote station 120a preferred antenna pattern based upon the antenna database isdetermined at Block 1420, and a remote station and the correspondingpreferred antenna pattern to communicate with are selected at Block1430. Based upon the antenna database and prior to communicating withthe selected remote station, it is determined at Block 1440 if anynon-selected remote stations have the potential of not being aware whensuch communications actually occurs. This is determined by comparing themeasured signal quality associated with the preferred antenna patternfor the selected remote station with the respective signal qualitiesassociated with the non-selected remote stations when using the samepreferred antenna pattern.

If there is a potential for a hidden node, then a message is broadcastat Block 1450 indicating that the access point 110 and the selectedremote station 120 are to communicate with one another. As noted above,this broadcast may be in the form of an unsolicited clear-to-sendmessage via the omni antenna pattern to the remote stations 120. The CTShas an unused address that does not correspond to any of the remotestations 120. Alternatively, a four-way frame exchange protocol (RTS,CTS, data and ACK) is performed with the selected remote station 120 toprevent the hidden node problem. The method ends at Block 1460.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. For instance, the access point isnot limited to the IEEE 802.11 standard. The antenna algorithm for anaccess point as discussed above is applicable to other types of localarea networks, as readily appreciated by those skilled in the art, suchas those defined by the IEEE 802.16 standard.

In addition, other features relating to antenna steering are disclosedin copending patent applications filed concurrently herewith andassigned to the assignee of the present invention and are entitledANTENNA STEERING FOR AN ACCESS POINT BASED UPON SPATIAL DIVERSITY, Ser.No. 10/870,719; ANTENNA STEERING FOR AN ACCESS POINT BASED UPON CONTROLFRAMES, Ser. No. 10/870,718; and ANTENNA STEERING AND HIDDEN NODERECOGNITION FOR AN ACCESS POINT, 10/870,702.

1. A method for operating an access point in a wireless local areanetwork (WLAN), the access point comprising a directional antenna forcommunicating with a plurality of remote stations, the directionalantenna including an omni angle and a plurality of directional angles,the method comprising: selecting one of the remote stations from theplurality of remote stations; transmitting a first probe signal via theomni angle of the directional antenna to the selected remote station;measuring a first probe response signal received via the omni angle fromthe selected remote station responding to the first probe signal;transmitting a respective second probe signal via each one of theplurality of directional angles of the directional antenna to theselected remote station; measuring a second probe response signalreceived via each directional angle from the selected remote stationresponding to the respective second probe signal; and storing in anantenna database the measured first probe response signal and therespective measured second probe response signals from the selectedremote station.
 2. A method according to claim 1 further comprising:selecting a preferred directional angle for the selected remote stationbased upon the measured second probe response signals; and comparing themeasured first probe response signal from the omni angle with themeasured second probe response signal from the preferred directionalangle.
 3. A method according to claim 2 further comprising selecting theomni angle or the preferred directional angle based upon the comparingfor continuing communications with the selected remote station.
 4. Amethod according to claim 3 wherein the preferred directional angle isselected if the measured signal associated therewith exceeds themeasured signal associated with the omni angle by a predeterminedthreshold.
 5. A method according to claim 1, further comprising:selecting a next remote station from the plurality of remote stations;repeating the transmitting of the first and second probe signals to thenext selected remote station, and the measuring of the first and secondprobe response signals received from the next selected remote station;storing in the antenna database the measured first probe response signaland the respective measured second probe response signals from the nextselected remote station; and repeating the selecting, the transmittingand the storing for each of the remaining remote stations from theplurality of remote stations.
 6. A method according to claim 1 whereinthe first probe signal comprises a request-to-send (RTS) message and thefirst probe response signal comprises a clear-to-send (CTS) message; andwherein the second probe signal comprises an RTS message and the secondprobe response signal comprises a CTS message.
 7. A method according toclaim 1 wherein the measuring comprises determining at least one of areceived signal strength indication, a carrier-to-interference ratio, anenergyperbit ratio and a signal-to-noise ratio.
 8. A method according toclaim 1 wherein selection of the omni angle and scanning through theplurality of directional angles are performed at the media accesscontrol (MAC) layer of the access point.
 9. A method according to claim1 further comprising updating the antenna database for the selectedremote station if there is no communications between the access pointand the selected remote station for a certain period of time, theupdating comprising repeating the transmitting of the first and secondprobe signals to the selected remote station, and the measuring of thefirst and second probe response signals received from the selectedremote station.
 10. A method according to claim 1 wherein the accesspoint is operating based upon at least one of an IEEE 802.11 standardand an IEEE 802.16 standard.
 11. A method according to claim 1 whereinthe directional antenna comprises at least one active element and aplurality of passive elements.
 12. An access point for a wireless localarea network (WLAN) comprising: a directional antenna comprising an omniangle and a plurality of directional angles; and a controller connectedto said directional antenna for control thereof, said controllerselecting one of the remote stations from the plurality of remotestations, transmitting a first probe signal via the omni angle of saiddirectional antenna to the selected remote station, measuring a firstprobe response signal received via the omni angle from the selectedremote station responding to the first probe signal, transmitting arespective second probe signal via each one of the plurality ofdirectional angles of said directional antenna to the selected remotestation, measuring a second probe response signal received via eachdirectional angle from the selected remote station responding to therespective second probe signal, and storing in an antenna database themeasured first probe response signal and the respective measured secondprobe response signals from the selected remote station.
 13. An accesspoint according to claim 12 wherein said directional antenna comprisesat least one active element and a plurality of passive elements.
 14. Anaccess point according to claim 12 wherein said controller comprises aphysical layer and a media access control (MAC) layer, and whereinselection of the omni angle and scanning through the plurality ofdirectional angles are performed at the MAC layer.
 15. An access pointaccording to claim 12 wherein said controller further selects apreferred directional angle for the selected remote station based uponthe measured second probe response signals; and compares the measuredfirst probe response signal from the omni angle with the measured secondprobe response signal from the preferred directional angle.
 16. Anaccess point according to claim 15 wherein said controller selects theomni angle or the preferred directional angle based upon the comparingfor continuing communications with the selected remote station.
 17. Artaccess point according to claim 16 wherein the preferred directionalangle is selected if the measured signal associated therewith exceedsthe measured signal associated with the omni angle by a predeterminedthreshold.
 18. An access point according to claim 12 wherein saidcontroller further performs the following: selecting a next remotestation from the plurality of remote stations; repeating thetransmitting of the first and second probe signals to the next selectedremote station, and the measuring of the first and second probe responsesignals received from the next selected remote station; storing in theantenna database the measured first probe response signal and therespective measured second probe response signals from the next selectedremote station; and repeating the selecting, the transmitting and thestoring for each of the remaining remote stations from the plurality ofremote stations.
 19. An access point according to claim 12 wherein thefirst probe signal comprises a request-to-send (RTS) message and thefirst probe response signal comprises a clear-to-send (CTS) message; andwherein the second probe signal comprises an RTS message and the secondprobe response signal comprises a CTS message.
 20. An access pointaccording to claim 12 wherein said controller updates the antennadatabase for the selected remote station if there is no communicationswith the selected remote station for a certain period of time, theupdating comprising repeating the transmitting of the first and secondprobe signals to the selected remote station, and the measuring of thefirst and second probe response signals received from the selectedremote station.